Large Asteroid to Pass Near Earth

On April 19th a considerable sized asteroid will pass about 4.6 lunar distances (1.8 million km) from Earth.  While there is no chance of it impacting our planet, this 650m asteroid was only discovered three years ago, and it will be the closest encounter of a large asteroid since asteroid Toutatis in September 2004. The next predicted fly-by of a large asteroid is 2027 with 800m wide 1990 AN10.

The expected magnitude could reach up to 11 during the close approach, hence a decent sized scope will be required, and due to the rapid movement may be hard to locate and track.

Sky chart for asteroid 2014 JO25 covering April 18th to 20th 2017

And as a bonus, comet PanSTARRS (C/2015 ER61) will also make its closest approach to Earth on the 19th, but 10 times farther away as the asteroid.  I should be visible with small telescopes or binoculars in the constellation Aquarius in the dawn sky.

Source: NASA/JPL

Could the Moon have its own moon?

I was reading the “Science & Vie” magazine when I came across a question from one of the readers: “Does the Moon have satellites?”  At first I considered this quite a silly question, but then realized that we have placed artificial satellites around the Moon.  So why could there not be natural ones?

The Moon is not billiard-ball smooth gravitationally.  It’s heavily scared surface due to past asteroid and comet impacts have affected the local density of the surface crust, and therefore the local gravity field varies across the surface of the Moon.

Map of gravity acceleration values over the entire surface of Earth's Moon.  Lunar Gravity Model 2011

Map of gravity acceleration values over the entire surface of Earth’s Moon. Lunar Gravity Model 2011

One of the famous effects of these local gravitational variations is the Apollo 11 landing, where Neil Armstrong had to take manual control to land, some 5km down range where the navigational computer was targeting.

Another factor is that any satellite around the Moon would also be under the influence of the Earth and the Sun.  Any asteroid captured by the Moon would quickly be ejected due to all these influences.  Now there are more favorable orbital angles: 27, 50, 76 and 86 degrees from the Moon’s equator.  But it would still be a highly unstable orbit.  All spacecraft that are placed in orbit around the Moon need to use up propellant to maintain orbit over time.  And when propellant is about to run out, most space agencies elect to purposely crash the satellite to obtain additional science data.  One recent example is NASA’s LADEE moon orbiter crashing on the Moon on April 18th, 2014.

In conclusion, no our Moon does not have any natural satellites, and if by chance it would capture a wandering asteroid, most experts believe it would only survive a century at most before impacting the lunar surface or getting flung out of orbit.